Anisotropic mechanical behaviour of Callovo-Oxfordian claystone under triaxial lateral unloading

Callovo-Oxfordian (COx) claystone has been selected as the host rock formation for the deep geological disposal of radioactive waste in France, called the Cigéo project. The excavation of drifts in the COx formation induced damage zones with an anisotropic shape, while the stress state around the drifts is almost isotropic. This is due to the anisotropic properties of the host rock formation and the instability caused by the brittle damage. In this study, the mechanical anisotropy of COx claystone was investigated through a triaxial shear test, where the axial stress was maintained constant while the lateral stress was decreased. Such a method was proposed for simulating one of the possible unloading paths involved during the excavation. The triaxial samples were prepared along different directions based on the angle between the axial loading direction and the one perpendicular to the bedding plane. Results show that the stress-strain curve exhibited an elasto-plastic pattern. With increasing deviatoric stress, a minor decline in Young’s modulus E was observed, suggesting progressive damage behaviour. The shear strength changed with increasing the loading angle, showing the anisotropic property of COx claystone. Moreover, the results in this study and collected from other works show a time-dependent behaviour of COx claystone. It is attributed to the coupled effect of creep and pore pressure dissipation inside claystone.